Extend StdIOService to take a working directory.

This will be required for pub support.
Extract GetSdkPath out into DartVS.Common.

We'll need this for Pub integration too.
Extract remote Sdk Location into constant.
Added a TODO about a multi-threading/race condition.
Removed unused cancellation token.

It's misleading to take a cancellation token if it's not used. We can
add this back in if we add support for it.
Return process exit code from StdIOService.
Remove onsolete constructor.

Pass SDK in for working dir in the cases where we don't really care.
Merge pull request #92 from DanTup/tidy-up

Tidy up (pub integration preperation)